kx500 stuck wide open

[chris91w]

i was wondering if anyone had any advise for me? i just rebuilt my 1999 kx500. (top end,reeds,etc). after the rebuild i started my bike and it was idolling a little high for like 15 seconds then without doing anything the motor pinned wide open. The kill switch wouldnt shut it down, pulling the spark plug cable wouldnt shut it down, and turning off the gas woulnt shut it down. the bike was pinned wide open for like 60 seconds with no gas and no sparkplug cable attached. In a pannic I had to slowly unscrew the spark plug until the motor lost compression. I was worried that the plug was going to shoot through the roof and my new motor was going to seize. Thankfully the motor shut down when it loss compression and the coolant wasnt steaming out so i dont think the cylinder got too hot. Anyone know where i went wrong. It was like the motor was possessed or something. I have done a rebuild on my bike before with no problems so i am clueless and confused! any help would be great. Thank you

[Brute]

Interesting. Motor requires fuel, air and combustion to run. So for fuel: surprised it was pulling enough fuel to run that high. That said, I have seen a diesel 'run away'. It takes a while to run a float chamber out of fuel which is probably why it was running for so long. I have in times past shut off the fuel and let my K5 run the fuel out of the float bowl. It is a BAD practice and takes a while for it to run out. I do not do it any more.
On the combustion, either something was 'glowing' in the cylinder or it was dieseling from really high compression. With self supporting combustion you are kind of screwed until it runs out of fuel, or you have to cut off the air to get it to shut down, or lower the compression to the point where it will not support combustion, as you did.
Have any air leaks in the engine? That could easily explain the RPM raising with the throttle closed. It would also get things hot in a hurry in the combustion chamber.
I am sure it was a hair raising experience!
 

[chris91w]

Ooh for sure it was totally scarey and I kinda panniced but that plug pull was my last resort. Thanks for the heads up as I always run the carb out of fuel before I store it. I'll also check for air leaks but doing top ends is pretty straight forward so I sure all my gaskets are seated but I'll definitely look close. Is the reeds a possible issue? I replaced them but that seemed straight foreword too. I'm not much of a mechanic and only tackle things I'm comfortable with so I don't know. Any advise is apprietiated.

[RoostDaddy]

In the event of a motor being stuck wide open, I've found the best way to kill it is pull the choke on if the kill switch doesnt do its job.  Also make sure you check your intake boot for any cracks this is the number one cause I've seen.  hope this helps.

[Gage]

+1 on choke with runaways. if all else fails, hold the Fbrake and stall it. Been there, scary stuff. Doesn't take long, Your lucky it didn't blow.

[wierdo]

same happened to me after a fresh rebuild except it started perfectly but during warm up and a couple of blips of the throttle it just started to rev on its own.anyway after s**tting myself it took me a while to sort the problem.
My problem was the reed gasket area but i could'nt tell if it was the gasket( new), the reed block(new V2's) or the rubber boot all individually looked in perfect condition.
when assembled though, the bolt pattern of the reed block doesn't put  much tension in the corners allowing air to be sucked in, problem was fixed with hi temp silastic to all mating surfaces and a thick bead on the outside that was then smoothed over the reed/gasket/boot area for a complete seal up.   

[chris91w]

you guys all gave me good sound advise and i checked everything. as to my last response i to thought it had to do with the intake boot that houses my reeds because i changed them when i did my top end. that being said and the fact that i was so freaked out and didnt want to start it again.... i took the bike in to the mechanic at the RACERS EDGE in simi valley california. They diagnosed and fixed my problem which turned out to be a stuck throttle cable because when i put the fuel tank back on i pinched the throttle cable which stuck it wide open. oops!!!!!!!!!!!!! TRE also said when the bike is wide open you cant shut the bike off when it is running at high rpm. so it was deiseling. crazy scarey and TRE only charged me 40.00 bucks to fix my F..K up.Freaking awsome of them. now my bike is perfect and ready for thanksgiving. thank you to everyone for your sound advise. Off to mojave now. Brrrrrrr